1. Field of the Invention
The present invention relates to a filter, and more particularly, to a reduced size comb filter.
2. Background of the Related Art
In general, a comb filter is provided with a differentiator and an integrator. If it is assumed that a down sampling ratio is "M" and a stage number is "N", a transmission function "H(Z)" of the comb filter may be expressed as follows. ##EQU1##
Thus, there is a differentiator and an integrator in a comb filter, and the present invention is related to the differentiator. A background art differentiator in a comb filter will now be described. FIG. 1 illustrates a hardware system of the background art differentiator having a down sampling ratio M="1" and a stage number N="5".
In the above transmission function, the differentiator transmission function is (1-Z.sup.-M).sup.N. If a down sampling is "1"(M=1) and a number of stages is "5"(N=5), the differentiator transmission function will be (1-Z.sup.-1).sup.5, which may be expressed by equation (1) as follows. EQU (1-Z.sup.-1).sup.5 =1-5Z.sup.-1 +10Z.sup.-2 -10Z.sup.-3 +5Z.sup.-4 -Z.sup.-5 (1)
A hardware system of the background art differentiator for implementing the differentiator transmission function of equation (1) is as shown in FIG. 1. The background art differentiator is provided with five subtractors 1 to 5 and five flipflops 6 to 10. A first flipflop 6 is for receiving and delaying a signal X(n), and a first subtractor 1 is for subtracting a signal delayed by the first flipflop 6 from the signal X(n). A second flipflop 7 is for receiving and delaying a signal from the first subtractor 1, and a second subtractor 2 is for subtracting a signal from the second flipflop 7 from the signal from the first subtractor 1. A third flipflop 8 receives and delays a signal from the second subtractor 2, and a third subtractor 3 subtracts a signal from the third flipflop 8 from the signal from the second subtractor 2. A fourth flipflop 9 is for receiving and delaying a signal from the third subtractor 3, and a fourth subtractor 4 for subtracting a signal from the fourth flipflop 9 from the signal from the third subtractor 3. A fifth flipflop 10 is for receiving and delaying a signal from the fourth subtractor 4, and a fifth subtractor 5 is for subtracting a signal from the fifth flipflop 10 from the signal from the fourth subtractor 4. When a master clock of the comb filter is 1/128 fs, a CLK-DIFF clock of 1/4 fs is applied to clock terminals on the flops 6-10.
The operation of the background art comb filter differentiator will now be described. FIG. 2 illustrates a timing diagram of a clock signal of the differentiator shown in FIG. 1.
Referring to FIG. 2, when the background art comb filter system has a master clock of 128 fs, a cycle Tm of the master clock is 1/128 fs and a cycle Tc of the CLK-DIFF is 1/4 fs. The signal X(n) is received in the order D1, D2, D3, D4, --with intervals of 1/4 fs. Since each subtractor conducts a subtraction only when the CLK-DIFF signal is received, the subtraction occurs in intervals of T1-T2, T33-T34 and the like. FIG. 3 illustrates a data flow in the background art differentiator. The differentiator forwards signals Y(n) at 1/4 fs intervals as shown in FIG. 3.
As described above, the background art comb filter differentiator has various problems. While a time period required for calculating one data is T1-T2 in the background art differentiator in a comb filter, all subtractors are idle in the rest of the time period T3-T33. Accordingly, the background art comb filter differentiator has a low efficiency. Further, one subtractor per stage is provided, which increases device size and costs.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.